Polishing a workpiece to produce a mirror-like, defect-free surface has applications in many fields of endeavor. Such polishing processes have become extremely important and widespread, for example, in the fabrication of semiconductor devices. The critical step of polishing a semiconductive wafer or substrate is required at a number of different stages along the varied processes employed to fabricate semiconductor devices.
Chemical mechanical polishing is rapidly becoming a technique of choice for polishing substrates, and particularly for use in the manufacture semiconductor devices on a wafer or substrate. The devices are connected by a process generally referred to as metalization, in which connecting lines of metal, often aluminum or copper, are applied by vacuum deposition or other suitable processes.
The performance level of semiconductor devices employing a conventional single metal layer connecting the devices is fast becoming unsuitable. Modern, high performance devices utilize multilevel metal interconnections. Multilevel connections may be constructed by depositing a dielectric or insulating layer over a first metal layer, etching via holes throughout the dielectric layer, and then depositing a second metal layer which fill the via holes to connect with the first metal layer. These devices offer higher device density and shortened interconnection lengths between the devices.
Since each of these metal and dielectric layers have an appreciable thickness, the wafer substrate is left with a non-planar topography as the various layers are patterned on top of one another. This type of non-planarity is often unacceptable in high density devices because the depth of field of the lithographic equipment that is used to print the smaller line width circuits on the wafer does not have a depth of focus sufficient to compensate for even small variations in wafer planarity.
In addition to the non-planarity caused by the fabricated device patterns, in-process wafer polishing, or planarization, must account for variations in overall wafer flatness as well. During the fabrication process, for example, the wafers may become bowed or warped.
In process polishing equipment, therefore, requires the specialized ability to achieve global, uniformly planar wafer surfaces in spite of these topographical wafer defects and variations. Chemical-mechanical polishing has gained wide acceptance as an effective means of achieving the global wafer surface planarity required by advanced devices employing multilayer metalization.
A typical chemical-mechanical polishing arrangement includes a wafer carrier having a generally circular pressure plate or carrier platen that supports a single substrate or wafer. A carrier film may be interposed between the carrier platen and the wafer. The wafer carrier is equipped with means to provide a downward force, urging the wafer against a polishing media (typically a circular pad), onto which is fed a polishing fluid. The polishing media is supported by a polishing platen. The polishing fluid may comprise a colloidal suspension of an abrasive and may also comprise a chemically reactive solution. A containment ring generally surrounds the wafer to prevent it from slipping off the carrier platen during polishing.
Typically, movement of the wafer relative to the pad, in the presence of the chemically reactive and/or abrasive polishing fluid and under pressure imparted by the wafer carrier, imparts a combination of chemical and mechanical forces to the wafer, the net effect of which is global planarization of the wafer surface. Generally, the polishing platen is rotatable as is the carrier platen. In a typical polishing apparatus, movement of the wafer relative to the pad is accomplished by rotating the polishing platen, the carrier platen, or both.
Rotating platen machines typically install a circular polishing pad and use it until the pad fails to obtain acceptable results because the pad becomes worn or becomes glazed with impacted polishing fluid and polishing particulate. At that time it is required to interrupt the polishing process and change the polishing pad. Other machines may use a rectangular pad or a continuous supply of polishing pad material that may be incrementally advanced over the polishing platen, to ensure that the polishing pad is never too worn to be effective.
Regardless of the configuration of the polishing pad, a common problem that occurs when the pad is not fixed to the polishing platen with an adhesive or other fixing means, is that the polishing pad migrates from its position when polishing forces are applied to it by the wafer carrier through the wafer. This migration results because the frictional forces between the wafer and the polishing pad, together with any chemical polishing media that might be employed, are greater that the frictional forces that exist between the polishing pad and the polishing platen. Such migration reduces the productivity and that accuracy of the polishing process requiring at least a reduction in the polishing pressure used in the process, thereby increasing the polishing time. Worse, the polishing pad may buckle during migration, resulting in nonplanar polishing results or total failure of the process (e.g. breakage of the substrate). These problems are not solely limited to chemical mechanical polishing but may also occur in purely mechanical polishing processes.
A problem that occurs particularly in chemical mechanical polishing machines is depletion of the chemical fluid or slurry between the substrate to be polished, and the polishing pad after a certain amount of polishing motion has occurred. Because of the relatively smooth and planar surfaces that comprise the polishing pad/platen and the substrate surface being polished, the polishing action tends to “sweep out” the chemical fluid/slurry and a vacuum or suction builds up between the substrate surface being polished and the polishing pad. Thus, this problem gets progressively worse with polishing time. Ironically, the problem also magnifies as the surface of the substrate becomes more planar and smooth, although the problem reduces the polishing efficiency and performance of the process.
It would be desirable to have an apparatus with the capability to prevent migration of the polishing pad, while at the same time allowing easy and quick replacement either continuously or intermittently. It would also be desirable to prevent the elimination of the chemical polishing agent, e.g., the phenomena known as “slurry starvation” between the substrate surface to be polished and the polishing pad.